It’s silent but deadly.All together, permafrost—which is anything below ground that stays frozen for two consecutive summers—traps around 1,300 billion tons of carbon dioxide and methane worldwide. As global temperatures rise, says Hugues Lantuit, an expert in coastal permafrost at the Alfred Wegener Institut in Germany, thawing permafrost “is estimated to be the second most important factor (after human activity) in delivering greenhouse gases to the atmosphere until 2100.”

It’s got history in its grip—for now.Permafrost preserves organic matter, such as feathers, bones and hair, in ancient inhabited sites. For an archeologist, that’s a dream. But studies show that by the end of the century, the active layer of permafrost—the part that thaws during the summer—will have completely disappeared, removing the insulation that keeps those artifacts intact. That’s why archeologists are scrambling to study ancient Inuvialuit sites in the Mackenzie Delta—including some spectacularly preserved driftwood houses—that are being lost to sea level rises, coastal erosion and thawing permafrost. (Forget pre-Inuvialuit sites, though—most of those are by now completely underwater.)

It’s messy. Yedoma, stretching across eastern Siberia, Alaska and part of the Yukon, is a type of permafrost that’s been frozen for up to 700,000 years, trapping fossilized organic matter from when giant ice age mammals roamed the Arctic (see next page). When that permafrost gets exposed, it’s like defrosting mystery goop that’s been in the back of your freezer since before you owned the freezer. And nobody wants to clean that up.